At temperature above water's critical point (for example, 374° C. and at 22.1 MPa/218 atm), water can behave as an adjustable solvent and can have tunable properties depending on temperature and pressure. Under such conditions, coal can be decomposed by hydrolysis, and cleavage products of coal can dissolve in the supercritical water, thus minimizing tar and char formation. This processing of coal can be performed within a supercritical water gasification reactor.